The ability of the motor system to respond appropriately in the presence of disturbances is a fundamental property known as impedance. Specifically, impedance relates a dynamic positional displacement to the corresponding torque about the joint. Many interesting studies investigating impedance have been published that elucidate numerous properties of motor control. However, there have been no studies that explore the dynamic impedance of lower-limb joints during dynamic tasks. In this study we propose to determine the dynamic impedance of the ankle during walking. We will use a custom built robotic platform to perturb the ankle about its center of rotation and measure the ground reaction force. Using mechanics, we can determine the torque about the ankle during the perturbation, which establishes the input (angular position displacement) and the output (torque) to be used in a system identification technique. We will employ a quasi-static, linear identification technique to determine the impedance of the ankle, subsequently analyzing the impedance in terms of mechanical components.